The invention relates to the dehairing or dewooling (depilating) of animal pelts, hides or skins, more specifically to depilatory paint and depilatory paint (depilation paint) thickener used in the depilation process in for example fellmongeries.
Depilation is a process whereby a depilatory paint is applied to a (sometimes pre-shorn) hide or skin and let to soak for several hours during which chemicals act to de-hair the skin. During that time, the paint should remain in place to provide for continuous chemical action, for which a certain thickness or viscosity is required. After depilation, the skins or hides (now often called slats) are washed and cleaned for further processing. Depilated hair is in general not further useful, depilated wool (now often called slipe wool) sometimes finds further use.
Traditionally (see depilatory thickeners, S. M. Cooper, New Zealand Leather and Shoe Research Institute) hydrated lime has been used for thickening depilatory paint. It has a number of advantages. It works well as a thickener because of its low solubility. It provides an alkali reserve for the depilation processes to keep the pH high enough for hair or wool loosening. It is readily available, and best of all it is cheap. Lime is still used for paint thickening in quite a high proportion of fellmongeries. However lime does have disadvantages when used as a thickener. In general, it does not stay in suspension. Furthermore, pelts painted with a low lime-paint tend to have a brighter grain, and they require less washing out of lime and are easier to delime. A number of alternative thickeners have been tried. Among the most successful are pregelatinised potato starch ethers. Pregelatinised potato starch ethers give good thickening at low concentrations of 1.5 to 3 percent. They are reasonably easy to mix into the paint, at least compared to some of the alternative non-lime thickeners, and they do not require any heating. However, in general they are not as easy to mix as lime. Unlike some of the non-lime thickeners, they are not so tacky that they cause sticking problems, with pelts sticking to each other or the boards of the pulling machines. They do not tend to have drip problems, and paint thickened with these thickeners is stable when sprayed under pressure. With some thickeners thinning under pressure leads drip problems. By using some lime in the system to act as an alkali reserve, sodium hydroxide is not needed to increase the pH. However there have been some problems in production. Paints thickened with these thickeners not always have behaved completely consistently when sprayed onto pelts, and this has caused problems of too much or too little paint on the pelts, or uneven coverage. This can lead to difficult pulling or residual wool problems or in some cases pelt damage.
It is thought that these inconsistencies are due to small changes in the viscosity of the paint from day to day, due to slight changes in the proportion of ingredients or the method of mixing, or even the temperature during mixing or application of the paint. Small differences in viscosity could lead to differences in application rate because a spray system is very sensitive to viscosity changes.
The operation of the depilation process can be looked at in two ways;
a) The Processes Taking Place in the Fellmongery
These are the observable processes that are carried out in the fellmongery on the skin and are controlled by the fellmongery manager. The process-steps involved may consist of; Wash; pre-flesh; squeeze or spin dry; apply depilatory paint; hold skins; remove wool.
Each of these processes are controlled to give good clean hair or wool removal with minimal damage to the skin.
b) The Underlying Processes
This requires an understanding of what is occurring as the depilatory paint is penetrating the skin, i.e.
how do sulphide and hydroxide penetrate the skin;
how much hydroxide and sulphide bind to the collagen in the corium layer;
how the sulphide and hydroxide react with the wool roots; the equilibrium between sulphide and hydrosulphide.
To date, depilation research has looked at the processes carried out on the skin and their improvement without looking at the underlying processes that are occuring in the skin. Approaching depilation from the view of the underlying processes will enable the critical processes to be determined. These processes can then be optimised to give better depilation performance.
The optimisation and control of these critical processes depend on the ability to observe what is occurring, both on the underlying processes and the processes carried out. Factors which are of importance during depilation are:
thickness of skin; paint performance; paint application rate; rate of penetration of chemicals.
Current techniques for application of depilatory paint involve applying a larger amount of depilatory paint to the centre of the skin. This is to compensate for the assumed 2:1 ratio between the thick and thin regions of the skin. However the actual ratio between the thick and thin regions of the skin may be much larger than assumed. This may mean that the differential spraying technique may not deliver enough paint to the thicker areas of the skin.
If the correct amount of paint at the correct concentration is applied to the skin, there may still be problems with depilation if the spray pattern does not hold. Common starch thickeners are currently used in the industry to hold the spray pattern. However, even with the addition of common starch thickeners it is often found that when the skins are stacked or pole hung that the spray pattern will shift. A correctly thickened paint should not produce such paint migration.
To ensure that paint migration does not occur, a depilatory paint must be thickened correctly with a suitable thickener. The means of determining the suitability of a thickener for depilatory paints has not always been well understood. Often the thickener used has relied on suggestions from chemical companies. The selection of a thickener by this means may not always meet-the requirements of depilation. To determine the suitability of a thickener for a depilatory paint, the requirements of depilatory thickeners must be understood. These requirements can be put into four categories:
1) Viscosity Requirements
For a thickener to be considered suitable, the thickener must provide the required level of viscosity consistently and simply. Special handling requirements limit the use of the thickener in the fellmongery situation. To satisfy this the thickener should;
Thicken at normal temperatures (20-25xc2x0 C.) without requiring heating.
easily disperse without lumping into sulphide solutions without the need for high speed agitators.
not be sensitive to shear.
2) Stability Requirements
Not only should a depilatory thickener provide the required level of viscosity, but it should also be stable, ie the thickener should not be broken down or form solid gels by process or chemical conditions. This can be satisfied if the thickener;
is not thinned excessively by mixing too long.
is stable under high NaOH and sulphide concentrations, e.g. does not gel or thin over time.
provides paints of consistent viscosity, e.g. will make the same paint again and again.
3) Tack Requirements
When two skins are placed together flesh to flesh after depilatory has been applied, there maybe some adhesion of the skins making pulling apart difficult. To be acceptable as a depilatory paint thickener, the thickener should not adhere skins when skins are placed together flesh to flesh for 2-3 hours.
4) Downstream Processing Requirements
The use of the thickener should not have any adverse effects on any further processing that may be carried out. The thickener;
should not affect the quality of the slats produced.
should not affect the quality of the slipe wool produced.
should not increase the waste treatment loading.
should easily be washed out.
The invention provides a depilatory paint thickener comprising a starch containing essentially only amylopectin molecules. Such a thickener is among others easily soluble in cold water; improves diffusion of chemicals (like sulphide or lime) throughout the skin; improves viscosity or resistance to shear of a paint, whereby less starch is needed to exert a similar effect; and increases the suspendability of a lime, allowing less lime to be used which creates environmental advantages in waste disposal.
Starches, both of the common variety containing both amylose and amylopectin, obtained from both cereals and tubers or roots and of the waxy variety, containing essentially only amylopectin molecules (e.g. 0-5% amylose), obtained from cereals, are widely used in foodstuff.
Common starch consists of two major components, an, in essence, linear xcex1(1-4)D-glucan polymer (branching is found at a low level) and a elaborately branched xcex1(1-4 and 1-6)D-glucan polymer, called amylose and amylopectin, respectively. Amylose has in solution a helical conformation with a molecular weight in the order of 104-105, or higher. Amylopectin consists of short chains of xcex1-D-anhydroglucopyranose units primarily linked by (1-4) bonds with (1-6) branches and with a molecular weight of up to 107, or higher.
Amylose/amylopectin ratios in native starches in plants are generally anywhere at 10-40 amylose/90-60% amylopectin, alsoxe2x80x94depending on the variety of plant studied. In a number of plant species mutants are known which deviate significantly from the above mentioned percentages. These mutants have long been known in maize (corn) and some other cereals. Waxy corn or waxy maize has been studied since the beginning of this century. Therefore, the term waxy starch has often been equated with amylose free starch, despite the fact that such starch was in general not known from other starch sources such as potato but mainly derived from corn. However, industrial use of an amylose free starch has never occurred in depilation processes in fellmongeries.
In a preferred embodiment of the invention a depilatory paint thickener is provided wherein said starch containing essentially only amylopectine molecules has been derived from a genetically modified plant. Amylose production in a plant is among others regulated by the enzyme granule-bound starch synthase (GBSS), which is involved in generating the amylose content of starch, and it has been found that many of the waxy cereal mutants described above lack this enzyme or its activity, thereby causing the exclusive amylopectin character of these apparently naturally genetically modified cereal mutants.
An example of a thickener provided by the invention is a starch obtained from an amylose-free potato plant which is for example lacking GBSS activity or GBSS protein altogether, thereby lacking amylose and having essentially only amylopectin molecules. In a preferred embodiment of the invention a depilatory paint thickener is provided wherein said starch from is derived from a genetically modified non-cereal plant, for example from a potato, banana, yam, canna or cassave. Genetic modification of non-ceraeal plants such as tuber or root plants is a skill available to the artisan, and for example involves modification, deletion of or insertion in or (antisense) reversion of (parts of) a gene, such as a gene encoding granule-bound starch synthase (GBSS), which is involved in determining the amylose content of starch. In order to manipulate such crop plants, efficient transformation systems and isolated genes are available, especially of potato, and others are found by analogy. Traits, such as absence of amylose, that are introduced in one variety of a crop plant can easily be introduced into another variety by cross-breeding. In the experimental part of this description a thickener is provided wherein said modified starch is obtained from a genetically modified potato, for example from a genetically modified potato plant variety. Examples of such a potato plant variety are the variety Apriori, Apropos or Apropect, or varieties derived thereof.
In a further embodiment of the invention a depilatory paint thickener is provided wherein said starch is a cross-linked starch, such as a epichlorohydrin cross-linked starch.
Crosslinking starch is in itself a method available to the artisan, various cross-linking agents are known, examples are epichlorohydrin, sodium trimetaphosphate, phosphorous oxychloride, chloroacetic acid, adipic anhydride, acrolein, dichloro acetic acid or other reagents with two or more anhydride, halogen, halohydrin, epoxide or glycidyl groups or combinations thereof which all can be used as crosslinking agents.
In a preferred embodiment a depilatory paint thickener is provided having a degree of cross-linking varying from 0.001% to 0.5%, preferably varying from 0.01% to 0.1%, more preferably varying from 0.025% to 0.05%, for example varying on whether the cross-linking occurs in solution or suspension. In the experimental part of this description a much preferred thickener is provided having a degree of cross-linking of between 0.025% to 0.05%, such as 0.033%, cross-linked in solution.
In yet another embodiment of the invention a depilatory paint thickener is provided wherein said starch is a stabilised starch, such as a hydroxyalkylated starch. Stabilisation by hydroxyalkylation or carboxymethylation of starch is for example obtained with reagents containing a halogen, halohydrin, epoxide or glycidyl group as reactive site. Chloro acetic acid (or its salt) is used as carboxymethylation reagent. In one embodiment of the invention said starch is stabilised by hydroxypropylation, hydroxybutylation, hydroxyethylation and/or carboxymethylation.
In a preferred embodiment of the invention a depilatory paint thickener is provided comprising a stabilised starch having a molar degree of substitution (MS) varying from 0.01 to 1.6, preferably from 0.1 to 1.2, more preferably from 0.2 to 0.8.
In a further embodiment, the invention provides a depilatory paint thickener wherein said starch is an pregelatinised or cold-water soluble or instant starch, providing easy solubility to a thickener. In general starch or starch derivatives are relatively insoluble in cold water. Viscosity and water binding is achieved by heating or cooking. For convenience starches are sometimes pre-gelatinised i.e. pre-cooked and dried. These starches are referred to as instant starches and perform without heating or cooking. Pre-gelatinisation can for example be achieved by spray cooking, spray drying, roll drying, drum drying, extrusion heating in aqueous water-miscible organic solvents or under high pressure.
A paint thickener as provided by the invention provides good and stable viscosity and shear resistance to a depilatory paint. Due to its high and stable viscosity and water binding properties, it can in general be used at lower concentrations than a thickener comprising a common starch with normal contents of amylose. It provides protection against abrasion of skins, especially during the stacking of the skins. It furthermore enables easier separation of stacked, limed pelts as it prevents drying out of the paint due to its high water binding properties.